The present invention relates to filters used in pneumatic circuits, and in particular to a self-cleaning filter intended mainly, but not exclusively, for filtering air.
It is known that various industrial plants use air to drain away processing wastes and/or for different kinds of treatments, and they include devices for filtering the air polluted by particles or substances of various origin. Reference will be made hereafter to filters employed in particular in the textile industries plants, being clear that the following is valid also for other similar applications.
It is known that conventional self-cleaning filters essentially consist of a drum housed in a case and covered with a continuous filtering element or baffle. This drum is provided with a relative motion with respect to a suction mouth located on the air inlet side and/or with respect to a blowing nozzle located on the opposite side. The relative motion is usually achieved by rotating the drum around mouths which are fixed or longitudinally shifting at most. Obviously, it is also possible to keep the drum still and rotate/shift the mouths.
The function of the suction mouth on the air inlet side is to suck up dust, wastes, etc. which deposit on said side of the filtering baffle. However, the effectiveness of said mouth is good only in case the filtering baffle is flat, so that the mouth can be positioned close to the baffle. On the contrary, in the most common case of pleated baffle (which increases the filtering surface with the same bulkiness) the bottom areas of the pleats are not adequately cleaned, thus reducing the advantage coming from the pleating.
In this case, in order to improve the baffle cleaning there is adopted a blowing nozzle, possibly rotating, which is usually located inside the drum. This nozzle intermittently emits a strong jet of compressed air which causes the dust to fall on the bottom of the filter case thanks to the counter-pressure. Such a cleaning system is based on the instantaneous shaking and on the temporary inversion of direction of the air flow through the baffle, however it has a limited effectiveness too.
In fact, first of all it can not be applied in a continuous way since it momentarily blocks the incoming air flow. Moreover, its action is not localized but general, whereby it is not particularly effective at the bottom of the baffle pleats where the suction mouth does not arrive.
A more effective type of filter is disclosed in US-A4.253.855, including a suction mouth and a blowing nozzle located adjacent to the baffle at mutually opposite positions and extending along the whole length thereof. This arrangement allows a better cleaning of the baffle but still is not completely satisfactory for particularly difficult types of filtration, e.g. the filtration of cotton fibers by means of a pleated baffle.